That is apparently a thing with elections where populists are running in a prominent position or with a high visibility concern. Italy at Berlusconi time was the same.. the polls always said no way he'd win and then he did. People vote populist in the secrecy of the voting cabin, they often are afraid/ashamed to own up to it when asked by, even anonymous, interviewers.

I don't know what precise result the polls were predicting, but I think that right before the election they were saying it would be close but that Hillary would win. In fact, she did gain more votes than Trump and if it had been simply on raw vote numbers, she would have won. However, there's this complicated electoral college things which just gave Trump the edge. Rather like our system can put a party in power with a minority of the votes cast.

If the figures a friend was quoting on Facebook were correct, it worked out that roughly 50% of Americans didn't vote. Just under 25% voted for Trump, just under 25% for Hillary and the odd couple of percent went to the third party candidates. So in this case, the polls possibly weren't all that wrong it's just difficult to be exact when things are so close. The same actually applies to Brexit.

Yes it was close- but it's in close elections that the work of pollsters comes into its own- and if they can't get it right there's no point. On the other hand If everybody knows there's going to be a landslide who needs a pollster?